lists.openwall.net   lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  PHC 
Open Source and information security mailing list archives
 
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:	Thu, 22 May 2014 22:21:25 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] workqueue: remove the unneeded cpu_relax() in __queue_work()

On Thu, May 22, 2014 at 9:47 PM, Tejun Heo <tj@...nel.org> wrote:
> On Thu, May 22, 2014 at 04:44:16PM +0800, Lai Jiangshan wrote:
>> When pwq->refcnt == 0, the retrying is guaranteed to make forward-progress.
>> The comment above the code explains it well:
>>
>>       /*
>>        * pwq is determined and locked.  For unbound pools, we could have
>>        * raced with pwq release and it could already be dead.  If its
>>        * refcnt is zero, repeat pwq selection.  Note that pwqs never die
>>        * without another pwq replacing it in the numa_pwq_tbl or while
>>        * work items are executing on it, so the retrying is guaranteed to
>>        * make forward-progress.
>>        */
>>
>> It means the cpu_relax() here is useless and sometimes misleading,
>> it should retry directly and make some progress rather than waste time.
>
> cpu_relax() doesn't have much to do with guaranteeing forward
> progress.  It's about giving a breather during busy wait so that the

This is not busy wait, the retry and numa_pwq_tbl() guarantee that
the retry will get a new pwq (even without cpu_relax()) as the comments says,
and the refcnt of this new pwq is very very likely non-zero and
cpu_relax() can't
increase the probability of non-zero-refcnt. cpu_relax() is useless here.

It is different from spin_lock() or some other spin code.

it is similar to the loop of __task_rq_lock() which also guarantees progress.

Thanks,
Lai

> waiting cpu doesn't busy loop claiming the same cache lines over and
> over ultimately delaying the event being waited on.  If you're doing a
> busy wait, you better use cpu_relax().
>
> Thanks.
>
> --
> tejun
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@...r.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Powered by blists - more mailing lists